1. Field of the Invention
The prevent invention relates to an image recording apparatus in which an image is formed by transferring a coloring material to a recording medium with a thermal head, particularly to the image recording apparatus which can perform high-speed printing.
2. Description of the Related Art
Recently, needs for print outputs of color digital images are grown as digital cameras widely spread. Various methods of printing the color digital image are proposed. For example, in a thermal transfer type printers with a thermal head such as a sublimation type printer, printing expression equivalent to a silver salt type print can be obtained by gray scale. Further, in the thermal transfer type printer, miniaturization can be achieved because solutions such as chemicals are not used, so that the thermal transfer type printer receives wide spread attention as the printer which has a potential for replacing the silver salt photograph at home.
FIG. 2 shows the conventional thermal head conducting control method. In the conducting method shown in FIG. 2, elements of the thermal head are heated by selectively passing current through the elements according to inputted image data, an ink ribbon is heated, and ink is transferred to the recording medium to form the image. At this point, for example in the case of 8 bits and 256-level gradation according the image data, coloring density varies in 256 levels by an energy varying method such as thermal head conduction time sharing and shading is applied, which results in the print having the photograph-like gradation. In this case, the image data varies from a 0 level to a 255 level. A 0-level gradation represents a white color. In the 0-level portion, it is determined that the image data does not exist. In a 255-level portion, it is determined that the image data represents the maximum density. In the case where the image data is the 0-level gradation, since the image data represents the white color, the current is not passed through the thermal head. On the other hand, the 255-level portion is maximum density, appropriate conducting pulses are applied to the thermal head, and thermal energy is conducted to the ink ribbon. Accordingly, the amount of transfer ink applied on the ink ribbon is controlled by changing the energy of the conducting pulses from the 1 level to the 255 level, and the image is formed in the recording medium which is of an image receiving member.
In the case where the image data is the 0-level gradation, since the image data represents the white color, the current is not passed through the thermal head. The 255-level portion is a maximum density portion, the appropriate conducting pulses are applied to the thermal head, and the thermal energy is conducted to the ink ribbon. Therefore, the amount of transfer ink applied on the ink ribbon is controlled by changing the energy of the conducting pulses from the 1 level to the 255 level except 0-level gradation, and the image is formed in the recording medium which is of an image receiving member. The amount of transfer ink varies according to the thermal energy given to the ink ribbon. At the same time, a base film of the ink ribbon is also stretched by the heat, and the base film is stretched. For example, when an image shown in FIG. 4 is printed, the 0-level portion which is of the non-conducting portion differs from the 255-level portion which is of the conducting portion in the stretch of the ink ribbon. Accordingly, the distortion shown in FIG. 5 is generated, and wrinkles tend to be generated in the ink ribbon by the difference in stretch. When the image is formed in the image receiving member, since the ink is not normally transferred in the wrinkle portion, the traces shown in FIG. 6 appears in the print, and the normal print is not performed. Particularly, in the case where the print is performed in a short time, the tendency is enhanced because it is necessary to apply the high energy in a short time, which results in ribbon wrinkle. The ribbon wrinkle seriously affects on quality of the print image. Therefore, in order to shorten the print time, it is necessary that the distortion is reduced.
In order to solve the ribbon-wrinkle problem, the method in which the conduction is performed to dots in a margin located outside the print area is proposed. However, the method cannot be provided for a frameless print (print without a margin) (for example, see Japanese Patent Application Laid-Open No. 11-301005).